Do away with ComposerWindowFactory

This is a reverts most of 52ad17238e3aa5cc842ce4695036acda21648852, the
motivation for which (exit after last window closed) is covered much
more directly by 3a8f6ff68a3ebc8bc6ab507df340e42d66cfd0f1 , meanwhile.

Besides simplification, this also fixes minor quirks, such as a new
composer window showing only a "CC", and no "To", if the user has composed
a reply, previously.